1. Field of the Invention
The present invention relates to a roof prism, and more specifically, to an injection-molded roof prism.
2. Description of the Prior Art
Referring to FIG. 1, there is shown a real image finder of the non-TTL (through the taking lens) type where a roof prism is used. This finder includes from the object side, an objective lens 1, a roof prism 2, a field frame 3, a penta prism 4 and an eyepiece 5. AX is a finder optical axis. Referring to FIG. 2, there is shown the finder of FIG. 1 viewed from the side of the field frame 3 along the X axis. The X, Y and Z axes are coordinate axes perpendicular to one another. As understood from FIGS. 1 and 2, a light beam on the optical axis AX is reflected at a ridgeline 2L.
The roof prism 2 shown in FIGS. 1 and 2 is made by injection molding, using a molding plate ma and a molding plate mb shown in FIGS. 3 and 4. FIG. 3 shows cross sections of the molding plates ma and mb taken on the XY plane along the ridgeline 2L of the roof prism 2 of FIGS. 1 and 2. FIG. 4 shows cross sections of the molding plates ma and mb taken on lines C1--C1 and C2--C2 of FIG. 3. In FIGS. 3 and 4, the roof prism 2 taken out of recesses Va and Vb of the plates ma and mb is shown so as to be viewed from the side along the boundary between a roof portion 2D and a side portion 2S (i.e. from the side of the cross sections of the molding plates ma and mb).
The injection molding of the roof prism 2 is performed in the following manner. First, the molding plates ma and mb are attached closely. Then, molten resin is injected through apertures (not shown) formed in the plates ma and mb into a cavity formed inside the plates ma and mb by the recesses Va and Vb. After the resin is cooled, the molding plates ma and mb are opened to take out the roof prism 2. The side portion 2S of the roof prism 2 is formed by the molding plate ma, while the roof portion 2D of the roof prism 2 is formed by the molding plate mb.
The roof prism 2 made in the above-described manner is attached to an attachment member 7 as shown in FIG. 5. The roof prism 2 is attached to the attachment member 7 in the following manner. Three pins 9 are provided to stand inside the attachment member 7. The roof prism 2 is inserted into the attachment member 7 so that one side (this surface is used as a reference for positioning along the Z axis) of the side portion 2S abuts the pins 9. Then, set springs 8x and 8y provided for positioning are attached to push the roof prism 2 along the X and Y axes. The side portion 2S of the roof prism 2 is pressed against reference attachment surfaces Sx and Sy for positioning along the X and Y axes.
With respect to the direction along the Z axis (i.e. the direction normal to both the ridgeline 2L of the roof prism 2 and the optical axis AX), the tips of the three pins 9 are used as the reference position along the Z axis, which is substituted for the reference attachment surface. The pins 9 are used so that the surfaces of the side portion 2S can easily be set at desired positions when the roof prism 2 is used with the side portion 2S being inclined. First, a set spring (not shown) is attached to a surface opposite to the surface of the side portion 2S which is in contact with the pins 9 while the side portion 2S is pressed against the tips of the pins 9. Then, by putting the lid (not shown) on so that the roof prism 2 is pushed along the Z axis, the positioning along the Z axis is completed. As described above, the side portion 2S of the roof prism 2 is conventionally used as a positioning portion for positioning along the Z axis.
In the injection molding of the roof prism, the injection of the resin and the taking out of the part are performed after two (sometimes more than two) molding plates are opened and closed as described above. However, since the position relationship between the molding plates are decided by a member such as a guide, the positions of the molding plates deviate from each other to some extent every time the molding plates ma and mb are opened and closed. This is caused, for example, by a backlash of a guide (not shown) used to open and close the molding plates ma and mb, and causes a deviation of positions of the molding plates ma and mb in the direction of arrow mz (FIG. 4).
Referring to FIG. 6, there is shown a roof prism 2 formed under a condition where the molding plates ma and mb are deviated from each other. In this roof prism 2, as shown in the figure, the roof portion 2D is deviated along the Z axis with respect to the side portion 2S serving as the positioning portion. Therefore, when this roof prism 2 is attached to the attachment member 7 as shown in FIG. 5, this deviation results in a position deviation of the ridgeline 2L along the Z axis with respect to the finder optical axis AX.
Referring to FIG. 7, there are shown the position of a finder optical axis AX1 when the ridgeline 2L is at the correct position, and the position of a finder optical axis AX2 when the ridgeline 2L is deviated from the correct position along the Z axis. When the finder optical axis AX1 is on the ridgeline 2L (i.e. when the finder optical axis AX1 is bent at the ridgeline 2L), the course of the finder optical axis AX1 is only bent at the ridgeline 2L at 90 degrees. However, when the ridgeline 2L of the roof portion 2D is deviated along the Z axis, the finder optical axis AX2 is deviated along the Z axis (toward the opposite side over the finder optical axis AX1) by an amount twice the deviation amount A of the ridgeline 2L at a roof surface 2d as well as its course is bent at 90 degrees. This means that the direction of observation through the finder changes.
Referring to FIG. 8, the broken lines show a condition where the roof prism is deviated along the Z axis under the condition of FIG. 2. In this case, the direction in which the finder faces is varied with respect to the finder optical axis AX as shown by the chain double-dashed lines. Since an index error is caused if the direction in which the finder faces varies as shown in FIG. 8, the area viewed through the finder differs from the area printed onto the film as shown in FIG. 9. As a result, it may occur that the face is not photographed although photographing of the whole body is intended. In FIG. 9, the solid line shows a photographic area, the broken line shows a finder field when the ridgeline 2L is at the correct position, and the chain double-dashed line shows a finder field when the ridgeline 2L is deviated from the finder optical axis AX, for example, due to a mis-attachment of the roof portion 2D.
FIGS. 7 to 9 show a case where the ridgeline 2L is deviated with respect to the finder optical axis AX due to the deviation of attachment position of the roof prism 2. This case is optically equivalent to a case where the roof prism 2 shown in FIG. 6 is attached to the attachment portion 7 (FIG. 5). Thus, if the ridgeline 2L is deviated with respect to the positioning portion, it is impossible to accurately decide the position of the ridgeline 2L with respect to the finder optical axis AX.